Solvent extraction of aromatic hydrocarbons with 1,2,3-tris-(2-cyanoethoxy)-propane

ABSTRACT

1,2,3-tris-(2-cyanoethoxy)-propane is used as a solvent for separation of aromatic hydrocarbons from hydrocarbon mixtures containing the same.

United States Patent Eberly, Jr. Jan. 14, 1975 1 SOLVENT EXTRACTION OF AROMATIC [56] References Cited HYDROCARBONS WITH UNITED STATES PATENTS 1,2,3-TR1S-(2'CYAN0ETH0XY)-PR0PANE 2,433,751 12/1947 Friedrnan 208/330 [75] Inventor: Paul E. Eberiy, Jr., Baton Rouge, 24441527 5/1948 La. 2,812,372 11/1957 Walsh et a1. 3,372,109 3/1968 Davis et a1...... [73] Assignee: Exxon Research & Engineering Co., 3,436,437 4/1969 Asaka et a1. 208/330 Linden, NJ. Primary Examiner1-1erbert Levine [22] Ffled' 1974 Attorney, Agent, or FirmJ. E. Luecke 21 AppL No.: 432,347

[57] ABSTRACT [52] US CL 208/330 260/674 SE 1,2,3-tris-(2-cyanoethoxy)-propane is used as a so]- [51 1m. 01 .1 Cl0g 21/20 vent for Separation of ammatic hydrocarbms from [58] Field of Search 208/330; 260/674 SE drocarbm Containing the same- 5 Claims, 1 Drawing Figure PATENTEI] JAN 1 4!.975

SOLVENT EXTRACTION OF AROMATIC HYDROCARBONS WITH l,2,3-TRIS-(2-CYANOETHOXY)-PROPANE BACKGROUND OF THE INVENTION This invention relates to procedures for separating aromatic hydrocarbons from hydrocarbon mixtures which consist of aromatic hydrocarbons admixed with other hydrocarbons species such as paraffins, branched paraffms, cycloparrafins and/or olefins.

Both extraction and distillation techniques have been employed in separating particular hydrocarbon species, e.g., the aromatic hydrocarbons, from petroleum hydrocarbon mixtures containing both aromatic and other compounds as listed above. It has been very difficult to obtain substantially complete separation employing distillation techniques where the hydrocarbon mixtures have narrow boiling point ranges. For such mixtures, solvent extraction techniques have been employed. These techniques are, however, not without their problems, one of the more significant being the difficulty in choosing a solvent which has both a high selectivity and a high solvent capacity for the aromatic hydrocarbon species to be separated as compared with those hydrocarbon species not desired. Most selective solvents, particularly those which are selective for aromatic materials will also dissolve significant proportions of non-aromatic hydrocarbon species.

In petroleum refining, it is desirable to treat the fractions in such a manner as to separate an aromatic rich stream from the saturated and olefinic aliphatic hydrocarbons. The aromatics have very high octane numbers and are useful for blending into motor gasoline. In addition, such aromatics as benzene, toluene and the xylenes are valuable feedstocks for a wide variety of uses in chemical industry. The raft'mates can be used as components in jet fuel or heating oils or as feed to catalytic reforming. Thus, over the years, there has been a continuing search for solvents which are selective to aromatic hydrocarbons only and have a high solvent capacity for said aromatic hydrocarbons and, at the same time, dissolve very little, if any, of the nonaromatic hydrocarbon species.

A number of materials have been proposed and described as selective solvents for the extraction of aromatic hydrocarbons from mixtures of aromatic and non-aromatic paraffinic, olefinic and naphthenic hydrocarbons. For example, US. Pat. No. 2,441,827, discloses the use of certain classes of monoand dinitriles for this purpose. The use of nitrile solvents is also mentioned in US. Pat. Nos. 2,458,067; 2,842,484; 3,372,109 and 3,436,437.

In recent years, sulfolane has found use as a selective extractant, either alone or in conjunction with other solvents to improve the selectivity of separation. There are, however, several drawbacks associated with the use of sulfolane as a selective solvent in hydrocarbon extraction processes. For example, in conventional solvent extraction processes, an extract phase containing the more readily soluble component is recovered by treating the starting mixture with the selective solvent and using a liquid-liquid extraction process. The solvent is thereafter recovered from the extract as the bottoms in a distillation operation. Sulfolane, however, readily degrades at its atmospheric boiling point. Therefore, it has been found necessary to use subatmospheric pressure in the separation of sulfolane from the remainder of.the extract phase. Furthermore, sulfolane has a relatively high freezing point -82F thus necessitating steam heating of the lines and equipment carrying the pure solvent in order to prevent its solidification. The necessity of using elevated temperatures when handling the pure solvent and the subatmospheric pressure required for the separation of the dissolved components from the sulfolane results in increased capital equipment costs and higher energy requirements.

It is, therefore, one of the objects of this invention to provide a solvent for selectively extracting aromatic hydrocarbons from mixtures of aromatic. olefinic and aliphatic hydrocarbons which will have a high degree of selectivity and capacity for the aromatics. Another object is to provide a solvent which has a high selectivity for aromatics and a low selectivity for olefins. It is also an object of this invention to obtain a solvent which is low in cost and can be used at more convenient temperatures and does not require the use of sub-atmospheric pressure in its recovery.

In fulfillment of the foregoing and other objects, it has been found that aromatic hydrocarbons can be extracted selectively from mixtures of aromatic, olefinic and aliphatic hydrocarbons using 1,2,3-tris-(2-cyanoethoxy)-propane as the extractive solvent. This solvent can be produced economically and in high yield by the cyanoethylation of glycerin as shown by 3CH=CH-CN CH2CHCH CH2-CHCH2 5 6 (CH2): (CHM (CH2)? ON. (IN 7 SN...

As a broad general class, suitable feedstocks for the satisfactory practice of this invention include fluid mixtures having a sufficiently high concentration of aromatic hydrocarbons to economically justify their recovery as a separate product stream. The present invention is particularly applicable to hydrocarbon feed mixtures which contain at least about 25 percent by weight of aromatic hydrocarbons. A suitable carbon number range for the feedstock is from about six carbon atoms per molecule to about 20 carbon atoms per molecule and, preferably, from about six to 10 carbon atoms per molecule. Typically, the feedstock will contain single ring hydrocarbons comprising a wide boiling mixture of benzene, toluene, and xylenes. These aromatic hydrocarbons are mixed with corresponding paraffins and olefins.

The aromatic hydrocarbons are separated from the mixed hydrocarbon stream by contacting the stream in a conventional liquid-liquid extraction technique, with l,2,3-tris-(2-cyanoethoxy)-propane.

The extraction of aromatic hydrocarbons from a mixed hydrocarbon stream using 1,2,3-tris-(2-cyanoethoxy)-propane may take place at temperatures from about to about 250F, preferably at temperatures ranging from about to 180F. The pressure is not critical and it is, therefore, convenient to use atmospheric pressure. Typically, from about 1 to 5 volumes, preferably about 2 to 3 volumes, of solvent are employed per volume of feedstock.

The invention may be better understood by reference to the appended drawing, which is a schematic representation of the apparatus for practicing the embodiment of theinvention.

A hydrocarbon feedstock containing aromatic hydrocarbons and non-aromatic hydrocarbons is introduced into the extractor 11 via line 12 at an intermediate point along the length of extractor ll. 1,2,3-tris-(2- cyanoethoxy )-propane, as the lean solvent, is introduced at the top of the extractor via line 13. Extractor 11 is a conventional liquid-liquid contacting device. A raffinate of low aromatic content is withdrawn from the top of the extractor by a line 14.

An aromatic rich solvent is removed from the bottom of extractor 11 by line 15 and sent to an extractive stripper 16 in which partial stripping of the aromatic hydrocarbons from the rich solvent occurs. The nonaromatic components, which are more volatile than benzene and the other aromatic components, are removed in the overhead stream via line 17. The stream is then condensed in condenser 18 and a condensed liquid is passed by means of line 19 into the separator receiver 20. Suitable conditions are maintained in separator receiver 20, including an adequate residence time, so as to permit the separation of aqueous and hydrocarbon phases. The aqueous phase is withdrawn from the bottom of separator receiver 20 via line 21 and further processed in a manner hereinafter described. The hydrocarbon phase comprising principally non-aromatic hydrocarbons and a residual amount of 1,2,3-tris-(2- cyanoethoxy)-propane solvent is withdrawn from separator receiver 20 via line 22 and refluxed to the extractor 11. The bottoms from the extractor stripper 16, comprising l,2,3-tris-(2cyanoethoxy)-propane solvent and aromatic hydrocarbons substantially free of nonaromatics, are removed via line 23 and passed to the extract recovery column 24.

In extract recovery column 24, operating conditions are maintained so as to separate the aromatic hydrocarbons from the solvent. This may be done by maintaining a temperature sufficient to vaporize the aromatic hydrocarbons but below the boiling point of 1,2,3-tris (2-cyanoethoxy)-propane. Alternately, the pressure in the extract recovery column may be maintained at subatmospheric level, for example, to 8 psi. The vaporized aromatic hydrocarbons are removed from the top of the extract recovery column via line 25, condensed in condenser 26 and then passed via line 27 to separator receiver 28. Suitable conditions are maintained in separator receiver 28, including an adequate residence time, so as to permit the separation of aqueous and hydrocarbon phases. The aqueous phase is withdrawn from the bottom of separator receiver 28 via line 29 and passed into wash tower 30. The hydrocarbon phase from separator receiver 28, which consists essentially of aromatic hydrocarbons, is passed via line 31 out of the system to a clay treater. The bottoms of extract recovery column 24, would consist essentially of lean solvent, is removed via line 32 and is then passed via line 13 into extractor 11.

The raffinate from the extractor, which has been removed via line 14 is passed to wash tower 30 where it is contacted with water to remove any dissolved 1,2,3-

45 Composition of Extract, Mol

tris (2-cyanoethoxy)-propane solvent. The raffinate is removed from wash tower 30 via line 33 and passed out of the system for further processing. The wash water from wash tower 30 is passed via line 34 into extract recovery column 24 in order to reclaim its content of 1,2- ,3-tris-(z-cyanoethoxy)-propane solvent. Similarly, the water which has been removed from separator receiver 20 via line 21 is passed to the extract recovery column' for the same purpose.

As noted above, one of the chief advantages in the use of l,2,3-tris-(2-cyanoethoxy)-propane is its extremely high selectivity for aromatic compounds. Selectivity is measured by the distribution coefficient, [3, which is defined as:

wherein X is the mo] fraction of non-aromatics, X is the mo] fraction of aromatics, and H and S represent respectively hydrocarbon-rich and solvent-rich layers.

The solvent of this invention, l,2,3-tris-( 2-cyanoethoxy)-propane has a B value which is considerably higher than that of sulfolane or of any other solvent in current commercial use. The following example is illustrative of the superior qualities of 1,2,3-tris-(2-cyanoethoxy)- propane as an aromatics extractant.

EXAMPLE 1 A naphtha obtained from a catalytic cracking unit and having a boiling range from 200 to 350F was extracted with 1,2,3-tris-(2-cyanoethoxy)-propane at room temperature using a solvent to naphtha ratio of l :1 and a contact time of 5 hours. Data for the distribution coefficient, [3, are shown in Table l and are com-' pared with results obtained from using sulfolane under indentical conditions.

Table l 40 Extraction of 200-350F. Cat Naphtha Solvent/Oil Ratio l Solvent Sulfolane Tris-CEP Composition of Raffinate, Mol

Saturates Unsaturates 73.8 7 l .l Aromatics 26.2 28.9

Saturates Unsaturates 18.] Aromatics 8L9 87.3 Distribution Coefficient, B 12.7 l

EXAMPLE 2 Table 2 Extraction of a 300-430F Naphtha at I42F and 2/1 Volume Ratio of Solvent to Oil Solvent I .2.3-Tris-(2-cyano- Tetraethylene N-Formylethoxy)-propane Sull'olane Gylcol morpholine Capacity of Solvent. Wt.% 5.7 l0.3 5.8 19.5 Composition of Extract (or Feed), Feed Mol Aromatics 65.3 96.3 90.1 89.5 76.0 Olefins 1|.3 2.1 7.2 6.7 ll.5

Table 2Cont1 r ued Extraction of a 300-430F Naphtha at 142F and 2/1 Volume Ratio of Solvent to Oil Solvent 1,2,3-Tris-(2-cyano- Tetraethylene N-Formylethoxy)-propane Sulfolane Gylcol morpholine Paraffins 12.4 0.7 1.2 1.8 6.5 Cycloparaffins 7.1 0.6 0.9 1.2 3.8 Condensed Naphthenes 3.9 0.4 0.6 0.7 2.1 Composition of Raffinate, Mol

Aromatics 58.7 49.5 58.8 41.6 Olefins 14.5 16.2 8.3 18.7 Paraffins 14.4 19.2 14.8 23.8 Cycloparaffins 8.1 9.9 13.6 10.5 Condensed Naphthenes 4.3 5.2 4.4 5.3 [3(aromatic) 18.1 9.3 5.95 4.45 B(olefin) 0.13 0.40 0.46 0.57

EXAMPLE 3 Heat stability experiments have been run in order to demonstrate the stability of l,2,3-tris-(2-cyanoethoxy)- propane. After 4 hours of heating at 400F, the solvent showed only a slight discoloration, whereas, at the same temperature, sulfolane turned black. It, therefore,

' appears that along with the use of 1,2,3-tris-(2-cyanoethoxy)-propane in the solvent extraction process, con- ;siderably higher temperatures can be employed than are currently possible using sulfolane.

What is claimed is:

l. A process for the separation of aromatic hydrocarbon compounds from mixtures of hydrocarbon compounds containing both aromatic and non-aromatic compounds, which comprises contacting in an extraction zone a mixture of aromatic and non-aromatic hydrocarbon compounds with l,2,3-tris-(2-cyanoethoxy)-propane to dissolve selectively the aromatics therein and form an extract phase comprising 1,2,3-tris-(2- cyanoethoxy)-propane and the aromatics and separating said extract phase from undissolved non-aromatics compounds.

2. The process of claim 1 in which the temperature in the extraction zone is from about to about 250F.

3. The process according to claim 1 in which the temperature is from about to 180F.

4. The process of claim 1 in which from 1 to 5 volumes of l,2,3-tris-(2-cyanoethoxy)-propane solvent is used per volume of hydrocarbon mixture.

5. The process of claim 1 in which from 2 to 3 volumes of 1,2,3-tris-(2-cyanoethoxy)-propane are used per volume of hydrocarbon mixture. 

1. A PROCESS FOR THE SEPARATION OF AROMATIC HYDROCARBON COMPOUNDS FROM MIXTURES OF HYDROCARBON COMPOUNDS CONTAINING BOTH AROMATIC AND NON-AROMATIC COMPOUNDS, WHICH COMPRISES CONTACTING IN AN EXTRACTION ZONE A MIXTURE OF AROMATIC AND NON-AROMATIC HYDROCARBON COMPOUNDS WITH 1,2,3TRIS-(2-CYANOETHOXY)-PROPANE TO DISSOLVE SELECTIVELY THE AROMATICS THEREIN AND FORM AN EXTRACT PHASE COMPRISING 1,2,3TRIS-(2-CYANOETHOXY)-PROPANE AND THE AROMATICS AND SEPARATING SAID EXTRACT PHASE FROM UNDISSOLVED NON-AROMATICS COMPOUNDS.
 2. The process of claim 1 in which the temperature in the extraction zone is from about 70* to about 250*F.
 3. The process according to claim 1 in which the temperature is from about 120* to 180*F.
 4. The process of claim 1 in which from 1 to 5 volumes of 1,2,3-tris-(2-cyanoethoxy)-propane solvent is used per volume of hydrocarbon mixture.
 5. The process of claim 1 in which from 2 to 3 volumes of 1,2,3-tris-(2-cyanoethoxy)-propane are used per volume of hydrocarbon mixture. 